The invention relates to a shaving apparatus provided with at least one pivotable cutting unit comprising an external cutting member and an internal cutting member which can be driven into rotation with respect to the former, a drive mechanism for driving the internal cutting member into rotation, and a coupling member comprising a coupling head and a coupling body, which coupling head can be detachably coupled to a coupling element of the internal cutting member, said coupling body being coupled to the drive mechanism, which coupling member has an axis of rotation and which coupling head is provided with curved drive surfaces between which curved connecting surfaces are situated, while, seen in a plane substantially transverse to the drive surface and containing the axis of rotation, each drive surface has a curvature with a first radius of curvature whose center lies substantially on the axis of rotation, and each connecting surface has a curvature with a second radius of curvature whose center coincides with the center of the first radius of curvature, said second radius of curvature being greater than said first radius of curvature, and the coupling element of the internal cutting member is provided with a cavity in which the coupling head can be accommodated and which is provided with drive surfaces for cooperating with the curved drive surfaces of the coupling head.
Such a shaving apparatus is known from U.S. Pat. No. 3,748,731. Seen in the direction of the axis of rotation, both the coupling head of the coupling member and the cavity of the coupling element of the internal cutting member have a rectangular shape with two long and two short sides. The two mutually opposed long sides of the coupling head form the curved drive surfaces, while the short sides form the curved connecting surfaces. The curved surfaces render it possible for the cutting unit to pivot in all directions (floating shaving heads), so that the cutting unit will satisfactorily lie against the skin during shaving. While driving the internal cutting member, the coupling head makes contact with the coupling element of the internal cutting member in two locations, i.e. the cooperating drive surfaces are in mutual contact in one location at any time. These contact locations are approximately diametrically opposed to one another with respect to the axis of rotation. This is referred to as a two-point coupling or drive. In practice, the coupling head will always fit the cavity of the coupling element with some clearance. A disadvantage of this coupling is that the coupling head can shift in the cavity of the coupling element in the direction of the connecting surfaces during driving. This is obviously dependent on the tolerances. At greater pivot angles, the coupling head is pulled to one side in the cavity, so that the coupling head no longer lies correctly centered in the cavity. The result is that the coupling head moves to and fro in the cavity during the rotary movement, which leads to stronger wear. More wear means more play, and accordingly a further deterioration of this aspect. The noise level will also increase as a result of this. It is attempted to keep this disadvantage as small as possible by making the tolerances of the coupling head and the cavity as narrow as possible. Narrow tolerances, however, imply a more expensive manufacture and more rejects.
It is an object of the invention to improve the coupling between the coupling member and the pivotable internal cutting member in a shaving apparatus described in the opening paragraph to the extent that the coupling is free from play during the drive.
To achieve this object, the shaving apparatus is characterized in that both the coupling head and the cavity of the coupling element have an approximately triangular shape which is rotationally symmetrical through 120xc2x0, seen in a direction along the axis of rotation.
During the drive operation, the coupling head makes contact with the walls of the cavity, which form the drive surfaces of the coupling element, in three locations. This leads to a so-called three-point coupling. The coupling head remains centered in the triangular cavity during the drive, also in the case of a comparatively great pivoting movement of the internal cutting member. Compared with the known coupling of U.S. Pat. No. 3,748,731, the coupling head according to the invention is allowed to fit the cavity with a comparatively large clearance. The mutually cooperating drive surfaces remain in contact with one another during the drive without the drive surfaces starting to slide along one another, at least as seen in a cross-section transverse to the axis of rotation. It is obviously possible for the drive surfaces to slide over one another in the direction of the pivoting movement of the internal cutting member.
In a preferred embodiment, each curved drive surface of the coupling head has a slight curvature, seen in a plane perpendicular to the axis of rotation. The slight curvature ensures that the point of contact between the cooperating drive surfaces will also lie on the curved drive surface of the coupling head and not on the edge between the drive surface and the connecting surface. The risk of wear is further reduced thereby.
A further embodiment is characterized in that the upper side of the coupling head is provided with a spherical bearing surface for cooperating with a bearing surface situated in the center of the cavity of the coupling element of the internal cutting member, which spherical bearing surface has a curvature with a radius of curvature whose center lies on the axis of rotation, which center, seen in the direction of the axis of rotation, is at a greater distance from the spherical bearing surface than is the center of the curved drive surfaces. In other words, the center of curvature of the spherical bearing surface lies below the center of curvature of the curved drive surfaces. A greater stability is achieved by means of this construction during operation of the shaving apparatus, so that substantially no vibrations will arise.
A yet further embodiment is characterized in that the coupling element is provided with sloping inlet surfaces for coupling to the coupling member. The sloping inlet surfaces facilitate the entry of the coupling head into the cavity of the coupling element.
The invention also relates to a cutting unit with an external cutting member and an internal cutting member which can be driven in rotation with respect to the former, comprising a coupling element provided with a cavity having a substantially triangular shape, designed for use in the shaving apparatus mentioned above.